When forming a coating film, e.g., a resist film, on a substrate, a method called spin coating is extensively used in that the spin coating can rapidly form the coating film. In the spin coating, the rear surface of a substrate is held in a spin chuck and a resist is supplied to a central portion of a front surface of the substrate. Then, the resist is diffused toward the peripheral edge portion of the substrate with a centrifugal force by rotating the substrate. A film is formed by drying the resist thus diffused.
In order to rapidly dry the resist, it is conceivable to increase the revolution number of the substrate to thereby accelerate volatilization of a solvent existing in the resist. However, the Reynolds number of an air flow on the substrate grows higher as the revolution number of the substrate becomes larger. If the Reynolds number exceeds a threshold value, the air flow on the substrate becomes a turbulent flow. Then, the turbulent air flow is transferred to the surface of a resist film. That is to say, coating spots, i.e., irregularities corresponding to the air flow are formed in the resist film. Consequently, the in-plane uniformity of a film thickness is reduced.
A semiconductor wafer (hereinafter referred to as a “wafer”) as a substrate is becoming larger in size. Use of a wafer having a diameter of, e.g., 450 mm, has been studied. If the wafer becomes larger in this way, the upper limit of the revolution number at which the generation of coating spots in the peripheral edge portion of the wafer can be suppressed becomes lower. In order to ensure that the coating spots are not generated even when the revolution number is made larger, studies have been made in which a baffle plate having a ring shape is installed on the wafer to prevent air flow from becoming turbulent at the lower surface side of the baffle plate.
In the meantime, it has been studied to perform formation of a resist film such that the amount of a resist supplied to a wafer becomes smaller. If the aforementioned baffle plate is installed when forming the resist film, as set forth in the evaluation tests to be described later, the film thickness of a peripheral end portion within the plane of the wafer becomes smaller than the film thickness of the remaining portion. As a result, there may be a case where the in-plane uniformity of a film thickness is reduced. In the related art, there has been proposed a technology, according to which a ring-shaped plate is installed above a square substrate so as to face the corner portions of the square substrate, thereby straightening an air flow on the substrate on which a resist is coated in the aforementioned manner. However, this technology does not mention the aforementioned problem and the solution to the problem.